The increasing demand to improve fuel economy, eliminate emissions, and reduce noise levels has driven the automotive market to develop a variety of propulsion mechanisms. As an alternative to the traditional internal combustion engine (ICE) powertrain the industry has developed a hybrid electric system powered by an electric traction motor(s) and an internal combustion engine. During varying driving conditions, hybrid electric vehicles (HEVs) will alternate between the separate power sources, depending on the most efficient manner of operation of each source.
A HEV may contain either a parallel drivetrain configuration or a series drivetrain configuration. Either of the configurations allows the ICE to perform relatively more efficiently than its conventional ICE counterpart. In a parallel hybrid vehicle the electric motor works in parallel with the ICE to combine the power and range advantages of the ICE with the efficiency and the electrical regeneration capability of an electric motor. The ICE drives the wheels through a transaxle. In a series hybrid vehicle, the ICE drives a generator to produce electricity for the electric motor, which drives the transaxle. This allows the electric motor to assume some of the power responsibilities of the ICE, thereby permitting the use of a smaller and more efficient ICE. An exemplary hybrid vehicle is described in U.S. Pat. No. 6,275,004 entitled “System for Battery Module Balancing via Variable Voltage DC/DC Converter In a Hybrid-Electric Powertrain, and U.S. Pat. No. 6,616,569 entitled “Torque Control System for a Hybrid Vehicle With an Automatic Transmission” both of which are incorporated by reference in their entirety.
In either parallel or series configuration, the electric motor/generator (MoGen) uses a combination of storage batteries to propel or drive the vehicle when the ICE is not operating. A hybrid vehicle will shut down the ICE during a stopped or idle condition, allowing the electric motor to propel the vehicle and eventually restart the ICE. Battery packs having secondary/rechargeable batteries are an important component of hybrid vehicle systems, as they enable the MoGen to store braking energy during regeneration and charging by the ICE.
To perform engine stop/start functions, relatively high electrical power is required to quickly crank the ICE from a stop. To achieve this function, a relatively higher voltage system (e.g. 36V) can be implemented for the MoGen bus, coupled with separate conventional 12V system to support the existing accessory loads. However, to reduce complexity, vehicle modifications, and possible component/system cost, a single voltage hybrid system can be considered. That single-voltage can be 12V or 42V, depending on the voltage/power requirement of a HEV and its accessories.
If a hybrid system is implemented into a conventional 12V electrical system, the voltage swings during dynamic Engine-Stop-Start (ESS) functions and engine stops (at idle) could be unsatisfactory. For example, when performing ESS functions, the large power draw results in large voltage swings. These voltage dips and surges result in customer distractions such as undulating headlights/dashlights and blower speed. These voltage swings can also present a performance issue if the battery or battery pack (already strained from ESS action) is needed to support accessories such as Electric Power Steering (EPS). Thus, for a single-voltage hybrid system to be viable, the present invention utilizes an additional power source consisting of either an auxiliary battery or an auxiliary battery combined with a DC/DC converter.